Rotary piston expansible chamber devices of the continuous apex contact type are known having arcuately spaced apices defining a plurality of expansible chambers as exemplified by the internal combustion engine popularly referred to as the Wankel engine, but such devices require special machinery and equipment to machine the epitrochoidal bore which forms the working chamber and need phasing gears to maintain the proper phase relationship between rotary piston and eccentric shaft in order to control the orbital motion of the rotary piston relative to the working chamber. Continuous apex contact rotary piston expansible chamber devices are also known of the type disclosed in such U.S. Pat. Nos. as 1,340,625 and 1,636,486 to LaPlanche wherein the rotary piston of a pump is spindle-shaped in cross section and has arcuately spaced apex seals and the bore cross section is a conchoid, but such devices require higher shaft speed than rotary piston speed and need a combination of gears to maintain the proper phase relationship between rotary piston, shaft and bore.